1. Field of the Invention
The present invention relates generally to the field of devices for supporting and transferring printed sheets from one printing press station to another, and more specifically to a transfer structure such as a cylinder wrapped in a soft netting material such as ultra-soft cotton fabric which is embossed or ridged, which is fit snugly around the cylinder, but not so tightly as to pull the ridging out of the netting material, for supporting and transferring a freshly printed sheet, the ridges serving to engage the sheet much like teeth while cushioning and yielding as the freshly printed surface of the sheet bears against the ridges to prevent smearing or other marring of wet ink, the cylinder preferably including a conveying surface with a matte finish to minimize slipping of the netting material relative to the conveying surface. A method is also provided of covering a conveying surface of a transfer structure such as a cylinder with yielding ridged material, and tightening the material to a tension level just short of pulling out the ridges, then causing the transfer structure to move relative to a printing press station in such a way as to pick up the sheet and carry it to another such station.
2. Description of the Prior Art
There have long been transfer structures for supporting and conveying freshly printed sheets of material from one station of a printing press to the next station. A key design objective has been for the structure to sufficiently grip the sheet to move it without smearing or smudging the wet ink on its surface.
One such prior transfer structure is that of DeMoore, U.S. Pat. No. 4,402,267, issued on Sep. 6, 1983. DeMoore teaches a method and apparatus for handling printed sheet material, including a skeleton transfer cylinder. The cylinder is coated with a fluorocarbon composite material to provide a cushioning effect for the sheets. The cylinder is wrapped in a piece of fabric which is secured loosely for the intended purpose of preventing the transfer of wet ink from one sheet to a successive sheet. The fabric is treated with an absorbance retarding substance such as SCOTCHGARD.TM.. A problem with DeMoore is that this structure is not well suited to small printing presses. Another problem is that loose, flopping fabric can bunch and wear quickly. The flopping fabric has a tendency of slapping the tail end of the impression cylinder and creating an ink build-up on the tail end of the transfer cylinder, and the netting thereon, which will cause marring of the surface of the next sheet of paper that is transferred.
Ellis, U.S. Pat. No. 1,231,141, issued on Jun. 26, 1917, discloses a fabric for printers' blankets and other purposes. Ellis describes a multiple layer blanket for packing impression cylinders of newspaper presses. The outer layer face is felt covered. It appears unclear whether the blanket material is intended to fit tightly or loosely over an impression cylinder. A problem with Ellis is that felt can mar a freshly printed surface. Another problem is that the multiple layer construction would likely be costly.
Goldsmith, U.S. Pat. No. 944,607, issued on Dec. 28, 1909, teaches tile coating machinery. Goldsmith includes a coating roll having a ribbed rubber periphery enclosed within a foraminous metallic sleeve. The foraminous sleeve is loosely mounted on the roll. A problem with Goldsmith is that the metallic sleeve would likely damage printed sheets and mar wet ink on their surfaces.
Boucher, U.S. Pat. No. 1,152,596, issued on Sep. 7, 1915, reveals a covering for use on a suction-roll which is used on paper making machines. The covering consists of a coarse knit sleeve, preferably knit from wool worsted yarn and adapted to be stretched over the suction roll. A problem with Boucher is that the coarse sleeve would likely be too rigid to convey freshly inked sheets without marring. Another problem is that these sleeves might slip on a typical smooth surfaced transfer cylinder.
Dickerson, U.S. Pat. No. 3,261,288, issued on Jul. 19, 1966, discloses an anti-smear jacket for a printing press transfer drum. Dickerson uses a series of elongated resilient spacing elements or strips on the outside of a transfer cylinder instead of fabric. A problem with Dickerson is that the hard, resilient strips could mar wet ink on a sheet.
McGibbon, U.S. Pat. No. 4,023,967, issued on May 17, 1977, reveals an electrophotographic liquid development method in which a uniform substantial interface contact is maintained. McGibbon uses a roller device having a core loosely circumscribed axially with a flexible sleeve, with a space between the core and the sleeve being substantially uniform. This space is filled with a curable plastic foam material to obtain a uniform contact and nip width when in rotational contact with cooperating surface. A problem with McGibbon is that the loose mounting of the sleeve on the core promotes marring and rapid sleeve wear. Another problem is that the resilient full sleeve contact can also increase the likelihood of marring.
An operation manual, entitled "Heidelberg M-Offset Multicolour-Perfector", dated October 1981, discloses a loosely mounted glass-bead blanket mounted on a transfer cylinder. A wire gauze material is used instead of a fabric. The problems presented by Heidelberg are essentially those of Dickerson.
Hartmann, U.S. Pat. No. 1,255,603, filed Feb. 3, 1916, discloses a printing cloth for presses where the cloth is tensioned around a printing roller or cylinder to convey printed stock. The outer conveying surface of the cylinder is grooved. A problem with Hartmann is that the cloth itself is not stated to be ridged or grooved to engage and convey the printed stock, which makes smearing and other marring likely.
It is thus an object of the present invention to provide a method and apparatus for transferring and supporting freshly printed sheets between printing press stations without marring the wet ink.
It is another object of the present invention to provide such a method and apparatus which is well suited for use on many of the smaller printing presses.
It is still another object of the present invention to provide such a method and apparatus which operate with minimal wear and maximum longevity.
It is finally an object of the present invention to provide such a method and apparatus which are reliable and produce a printed sheet product of consistent high quality.